Thermodynamic properties of the aggregation behavior of a dicationic ionic liquid determined by different methods

Colloids and Surfaces A: Physicochem. Eng. Aspects 494 (2016) 1–8

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Thermodynamic properties of the aggregation behavior of a dicationic ionic liquid determined by different methods Clarissa P. Frizzo a,∗ , Caroline R. Bender a , Paulo R.S. Salbego a , Gabrielle Black a , Marcos A. Villetti b , Marcos A.P. Martins a a Núcleo de Química de Heterociclos (NUQUIMHE), Department of Chemistry, Federal University of Santa Maria (UFSM), CEP 97105-900 Santa Maria, RS, Brazil b Laboratório de Espectroscopia e Polímeros (LEPOL), Department of Physics, Federal University of Santa Maria (UFSM), CEP 97105-900 Santa Maria, RS, Brazil

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• The effect of temperature on the •

• • •

aggregation of a dicationic IL was evaluated. Thermodynamic parameters of aggregation were determined by conductivity and NMR measurements. Surface tension investigation showed that surfactant activity decreased at higher temperatures. DLS studies showed that aggregates size decreased at higher temperatures. The aggregation of dicationic IL is enthalpy-driven.

a r t i c l e

i n f o

Article history: Received 18 September 2015 Received in revised form 6 January 2016 Accepted 11 January 2016 Available online 13 January 2016 Keywords: Dicationic ionic liquid Thermodynamic parameters NMR DLS Conductivity Surface tension

a b s t r a c t The temperature effect on the aggregation of the IL 3,3 -(octane-1,8-diyl)bis(1-methyl-1H-imidazol-3ium)bromide ([BisOct(MIM)2 ]Br2 ) in aqueous solution was studied in order to determine the aggregation properties, thermodynamic aggregation parameters, and aggregate size. The aggregation was investigated via conductivity, surface tension, dynamic light scattering (DLS), and nuclear magnetic resonance (NMR). The critical aggregation concentration (CAC), free energy of aggregation (G◦ agg ), and ionization degree (˛) increased significantly with the increase in temperature. It was observed that the aggregation mechanism of [BisOct(MIM)2 ]Br2 is governed primarily by the enthalpic factor (H◦ agg ). The entropy term (TS◦ agg ) becomes relevant above 318.15 K. The free energy adsorption (G◦ ads ) data demonstrated good surfactant activity for the dicationic IL studied; however, this property decreased at higher temperatures. © 2016 Elsevier B.V. All rights reserved.

1. Introduction ∗ Corresponding author. Fax: +55 5532208756. E-mail address: [email protected] (C.P. Frizzo). http://dx.doi.org/10.1016/j.colsurfa.2016.01.015 0927-7757/© 2016 Elsevier B.V. All rights reserved.

The aggregation behavior of gemini imidazolium ionic liquids (ILs) is investigated due to the promising characteristics that

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Millipore quality water (Elix-03 from Barueri, Brazil; and Milli-Q from Molsheim, France) was prepared in our lab. 2.2. Synthesis

Fig. 1. Chemical structure of [BisOct(MIM)2 ]Br2 .

have resulted from this process [1]. The use of dicationic ILs includes thermoregulated applications such as liquid crystal formation [2], metal coating [3], catalysts in organic reactions [4], separation/extraction agents [5], and electrolytes for energy storage devices [6]. Thus, the study of the thermodynamic aggregation parameters is required and can be done via numerous techniques, such as conductivity, surface tension, and fluorescence [1]. The data obtained in aggregation studies are mainly the critical aggregation concentration (CAC) and tensiometric parameters [7–9]. However, an understanding of the thermodynamic parameters of IL aggregation, such as enthalpy (H◦ agg ), entropy (S◦ agg ), and free energy of aggregation (G◦ agg ) at various temperatures, enables the determination of the contribution of hydrophobic (van der Waals interactions) and electrostatic effects from the ILs in the aggregation mechanism [10,11]. Some works have shown thermodynamic parameters for different ILs, mainly through variation of the cation type and/or size of the alkyl chains [9,10,12]. The temperature of the system is an important variable for influencing the self-assembly of ILs in solution. An increase in the temperature strongly affects the cation/anion, cation/solvent, and anion/solvent interactions, thereby increasing the CAC and G◦ agg values and decreasing the counter-ion binding degree (ˇ). Singh and Kumar [13] and Li et al. [14] investigated the self-aggregation of ILs in aqueous solution using conductivity and surface tension. They showed that aggregation generates conformational changes in different ILs. However, to the best of our knowledge, studies showing 1 H NMR operating in a defined temperature range in order to determine thermodynamic parameters of specific dicationic IL aggregates have not yet been reported. Recently, we showed the importance of the spacer chain length for the aggregation of dicationic ILs in water [15]. This work encouraged us to investigate the aggregation behavior of dicationic ILs with different anions. The study of anion variation was surprising since special anions promote the emergence of distinct interactions that may totally modify the aggregate’s structure [16]. After the elucidation of the aggregation process for several types of dicationic ILs, we began looking toward more advanced studies and seeking a deeper understanding of these physicochemical systems formed by ILs with short side alkyl chains. Thus, the aim of this work was to investigate the effect of temperature on the aggregation of [BisOct(MIM)2 ]Br2 (Fig. 1) in aqueous solution, using 1 H NMR, conductivity, surface tension, and DLS. Knowing this information enables determination of thermodynamic aggregation parameters and helps in the understanding of thermoregulated applications.

2. Materials and methods 2.1. Materials The 1,8-dibromooctane (98 wt%), methylimidazole (99 wt%), deuterium oxide (99.9 atom% D), chloroform-d (99.96 atom% D), and tetramethylsilane (ACS) were purchased from Sigma–Aldrich (St. Louis, MO, USA). The acetonitrile and ethyl ether (HPLC) were purchased from Tedia (Rio de Janeiro, RJ, Brazil). All chemicals were high-grade purity products and were used as received. Deionized

The IL was synthesized in our laboratory in accordance with the methodology developed by Shirota et al. [17]. The structure was confirmed by: 1 H and 13 C NMR spectra recorded in DMSO-d6 on a Bruker Avance III (1 H NMR at 600 MHz) at 298.15 K; differential scanning calorimetry using an MDSC Q2000 (T-zeroTM DSC technology, TA Instruments Inc., USA); and electrospray ionization mass spectra (ESI-MS) with a Triple Quadrupole 6460 LC/MS-MS (Agilent Technologies, USA) operating in the positive-ion mode. The data was in accordance with data previously described by our research group [15,16]. 2.3. Preparation of aqueous solution with IL Aqueous IL solutions were prepared by weighing the IL in a volumetric flask using an analytical balance with a precision of ±0.001 g (Marte AL 500, Brazil). The volumetric flask was filled with doubly distilled and deionized Millipore-quality water (Elix-03 from Barueri, Brazil; and Mili-Q from Molsheim, France). From this solution, stock solutions of IL were prepared and further diluted to yield concentrations in the range of 25–325 mM, 25–500 mM, and 25–1000 mM for conductivity, surface tension, and NMR, respectively. 2.4. Conductivity measurements Conductivity was measured using a conductivity meter (DIGIMED CD-21, São Paulo, Brazil) at 288.15, 298.15, 308.15, and 318.15 K—the temperature was controlled by a thermostatic bath. The cell was calibrated with aqueous KCl solution (0.01 mol L−1 ), and a cell constant of 1.41 mS cm−1 was determined at 298.15 K. 2.5. Surface tension measurements Surface tension measurements were done using the Donoüy ring method. Solutions with concentrations of 25–500 mM were prepared. The analyses of the surface tension of aqueous IL solutions were done at 288.15, 298.15, and 308.15 K. The equipment used was a tensiometer Krüss GmbH K20 Easy Dyne (Hamburg, Germany) coupled to a Julabo F12 refrigerated/heating circulator (Seelbach, Germany). 2.6. Dynamic light scattering (DLS) Dynamic light scattering measurements were performed in the temperature range of 298.15–318.15 K, using a multi-angle dynamic and static light scattering system (Brookhaven Instruments Corporation, NY, USA) with a mini L-30 laser (35 mW), operating at a fixed angle of 90◦ . Distributions of relaxation times were obtained from the intensity time-autocorrelation function, by using the GENDIST program which employs the REPES algorithm. In this study, the distributions of the relaxation times are shown as A() versus log  (␮s). The hydrodynamic radius (Rh ) of the aggregates was calculated through the Stokes–Einstein relation [18]. IL solutions of 300 mM were prepared with Milli-Q water previously filtered in a nylon filter with 0.45 ␮m sized pores. 2.7. NMR measurements 1H

and 13 C NMR spectra were recorded on a Bruker Avance III at 600.13 MHz and 13 C at 150.03 MHz) equipped with a BCU II unit as cooling/heating system for the probe (temperature range

(1 H

C.P. Frizzo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 494 (2016) 1–8

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Fig. 2. Plot of electrical conductivity versus IL concentration for [BisOct(MIM)2 ]Br2 in water, at 298.15 K.

of 193.15–333.15 K). The samples were placed in 5 mm tubes with D2 O, and a capillary containing TMS diluted in CDCl3 was added as an external reference. Samples with different IL concentrations were evaluated at 288.15, 298.15, 308.15, and 318.15 K.

aggregation can be derived by applying Eqs. (2) and (3), respectively [24].

 

◦

Hagg =

3. Results and discussion ◦

The study of the effect that temperature has on the aggregation behavior of [BisOct(MIM)2 ]Br2 was investigated through conductivity, surface tension, DLS, and NMR—the results for each method will be discussed separately and then a comparison will be presented of the thermodynamic aggregation parameters. 3.1. Thermodynamic aggregation parameters (NMR and conductivity) The CAC values of [BisOct(MIM)2 ]Br2 were evaluated from 288.15 to 318.15 K using the conductivity method. The plot of electrical conductivity () versus concentration for this IL in water, at 298.15 K, is shown in Fig. 2 (plots of other temperatures are depicted in Fig. S1 of the Supporting information). Normally, the CAC value is measured at the point where the two linear fragments intersect [19]; however, a CAC value can be more precisely determined when Carpena’s method is used [20]. The method is based on the fit of the experimental data for a function obtained by direct integration of a Boltzmann type sigmoidal function. In this work, the CAC and degree of ionization (˛) [21] were obtained using Carpena’s method [20]. The correlation coefficient (r) for the curves obtained by this method had satisfactory values (Table 1). The aggregation process was described by the mass action model, in which aggregation is treated as the equilibrium between monomers and aggregates. The G◦ agg of [BisOct(MIM)2 ]Br2 in water was determined from the following equation [22,23]: ◦





Gagg = RT 0.5 + ˇ ln xCAC

(1)

in which xCAC is the CAC in terms of mole fraction, and the degree of counter-ion binding (ˇ) is given by 1 − ˛. Degree of ionization (˛) was obtained from the ratio of the slopes above and below the break indicative of the CAC. Once the G◦ agg as a function of temperature is known, the standard enthalpy (H◦ agg ) and entropy (S◦ agg ) of

Sagg =

◦

∂ Gagg /T



∂ 1/T ◦





(2)

◦

Hagg − Gagg T

(3)

The thermodynamic parameters at each temperature, evaluated from the conductivity measurements, are listed in Table 1. From Table 1, it can be seen that the CAC and ˛ values increase upon raising the temperature. The effect of temperature on the CAC value for monocationic IL can be understood by considering two main contrasting aspects. The higher temperature intensifies the hydration degree of the hydrophilic head groups, which facilitates aggregate formation [25]. The temperature increase provides more energy to the system and, consequently, intermolecular interactions are broken more easily and the hydrophobic chains are surrounded by the water molecules, which makes aggregation more difficult [1]. This fact is demonstrated by the increase in the CAC value and decrease in the equilibrium aggregation constant (Kagg ) values. Furthermore, lower stabilization energy is observed in the form of less negative G◦ agg values. Similar to monocationic ILs, at lower temperatures the bromide ions in dicationic ILs are more strongly bound at the aggregate’s surface, thus reducing more effectively the repulsive interactions between the head groups and favoring the aggregation [26]. Furthermore, it was reported for monocationic ILs that larger aggregates had a major tendency to attract counter-ions [27,28]. Thus, in the case of dicationic ILs, it can be speculated that the aggregates are larger at lower temperatures than at higher temperatures and are more capable of attracting counter-ions. This supposition is confirmed by the ˇ values and the Rh values obtained by DLS measurements (see Table 3)—both values increase with the decrease in temperature. The G◦ agg values were negative at all the temperatures evaluated, which indicate that aggregation is thermodynamically favored. The negative H◦ agg values are what mainly contribute to the G◦ agg and they indicate that the aggregation mechanism of [BisOct(MIM)2 ]Br2 in water is enthalpy-driven for all temperatures. Moreover, this fact indicates that [BisOct(MIM)2 ]Br2 behaves differently to other dicationic imidazolium-based ILs with long

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Fig. 3. Chemical shifts (ıobsd ) observed in the H31 versus 1/C plot for [BisOct(MIM)2 ]Br2 in water, at 288.15–318.15 K.

Fig. 4. Thermodynamic aggregation parameters as a function of temperature for [BisOct(MIM)2 ]Br2 , using (a) NMR and (b) conductivity data. The squares, circles, and triangles correspond to G◦ agg , H◦ agg , and TS◦ agg , respectively.

Table 1 Thermodynamic parameters obtained from conductivity measurements of [BisOct(MIM)2 ]Br2 in water. T (K)

CAC (mM)

˛

G◦ agg (kJ mol−1 )

H◦ agg (kJ mol−1 )

S◦ agg (kJ mol−1 K−1 )

−TS◦ agg (kJ mol−1 )

Kagg

ra

288.15 298.15 308.15 318.15

118 125 139 163

0.35 0.39 0.43 0.48

−16.81 −16.62 −16.36 −15.57

−22.31 −23.40 −32.52 −40.54

−0.019 −0.023 −0.052 −0.078

5.49 6.78 16.16 24.97

1116.59 817.08 593.00 360.49

0.997 0.996 0.991 0.998

a

Correlation coefficient of the curves obtained via Carpena’s method.

Table 2 Thermodynamic parameters obtained by NMR measurements of the aggregation process for [BisOct(MIM)2 ]Br2 . T (K)

CAC (mM)

G◦ agg (kJ mol−1 )

H◦ agg (kJ mol−1 )

S◦ agg (kJ mol−1 K−1 )

−TS◦ agg (kJ mol−1 )

ra

288.15 298.15 308.15 318.15

463 468 476 493

−12.87 −12.75 −12.62 −12.37

−16.33 −16.43 −18.43 −20.33

−0.0120 −0.0123 −0.0188 −0.0250

3.46 3.68 5.81 7.96

0.994 0.992 0.996 0.989

a

Correlation coefficient for the curves obtained via Carpena’s method.

alkyl side chains and long spacers. Ao et al. [26] for example, verified that the aggregation of [bis-AlkylDoDecIM]Br2 (Alkyl = Et, Bu) is governed primarily by the entropy, while the aggregation of

[bis-HexDoDecIM]Br2 is enthalpy-driven at lower temperatures, and entropy-driven at higher temperatures. On the other hand, the aggregation mechanism of [BisOct(MIM)2 ]Br2 is similar to

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Table 3 Hydrodynamic radius (Rh ) of [BisOct(MIM)2 ]Br2 aggregates obtained from DLS measurements at different temperatures. Temperature (K)

Rh (nm)

298.15 308.15 318.15

236 183 175

pyrrolidine-based dicationic ILs [BisBut(Cn py)2 ]Br2 (n = 10, 12 and 14), which have enthalpy-driven aggregation behavior. Zhang et al. [19] showed that H◦ agg changes drastically when the temperature is raised—this mainly occurs through a combination of the hydrophobic effect and electrostatic repulsion, which controls the aggregate formation [10,11]. The 1 H NMR spectra of monocationic ILs at different concentrations indicate changes in the chemical shifts of different 1 H nuclei before and after aggregation [29,30]. The magnitude and nature of the shifts is reflected in the differences in the chemical environment felt by the hydrogen and its involvement in the aggregation process [13]. Upon aggregation, all the hydrogen atoms of [BisOct(MIM)2 ]Br2 showed an upfield chemical shift. The upfield shift of the hydrogens from the hydrophilic region is a consequence of the diamagnetic anisotropy of the imidazolium rings that are stacking via ␲· · ·␲ interactions. On the other hand, the upfield chemical shift from the hydrophobic region may be related to the decrease in the polarity of the medium after aggregation (see Fig. S2 in Supporting information). The change in the chemical shift (ıobsd ) of the methyl group’s hydrogen bonded to the imidazolium ring (H31) of [BisOct(MIM)2 ]Br2 was monitored as a function of concentration and was used to determine the CAC at different temperatures. The chemical shift (ıobsd ) of H31 versus the 1/C plot, at 288.15–318.15 K, is depicted in Fig. 3. Similar to conductivity, the aggregation process was described by the mass action model. The ˇ values used in Eq. (1) to obtain both G◦ agg by NMR and conductivity, were obtained by conductivity data. The thermodynamic parameters and CAC values obtained by 1 H NMR can be seen in Table 2. It can be seen that at lower concentrations (higher 1/C values), ıobsd is almost constant and then quickly changes to lower values with the increase in the concentration (lower 1/C values), which indicates that the aggregates have been formed. The CAC values are obtained by applying Carpena’s method [20] from the plots of ıobsd versus 1/C. The CAC values found through NMR are higher than the values found through conductivity. This fact has already been observed in the literature for other ILs [13]. At 273.15 K, the CAC values of [BisOct(MIM)2 ]Br2 were different to those reported in the literature [15]. The disagreement in these values can be related to the different techniques applied to detect the CAC region, which measure distinct physicochemical properties of the solutions. As observed in the conductivity data, the G◦ agg obtained from NMR data indicated a spontaneous enthalpy-driven process (Table 2). The TS◦ agg term becomes more important at higher temperatures. The thermodynamic parameters found through NMR demonstrate the same behavior as the parameters found through conductivity; however, their absolute values are not similar (Fig. 4). Additionally, the dependence of the chemical shift of the methyl group (H31) on the concentrations was used to determine the aggregation number (Nagg ). The calculations were based on the mass action law using Eq. (4), in accordance with Zhao et al. [31]. The plot for log[mt (ıobsd − ımon )/(ımic − ımon )] versus log[mt (ımic − ıobsd )/(ımic − ımon )] is depicted in Fig. 5.



     ıobsd − ımon ımic − ıobsd  = Nagg log mt   + logKm (4) log mt  ımic − ımon

ımic − ımon

Fig. 5. Plot of log[mt (ıobsd − ımon )/(ımic − ımon )] log[mt (ımic − ıobsd )/(ımic − ımon )], at different temperatures.

versus

Fig. 6. Size distribution via DLS for the IL [BisOct(MIM)2 ]Br2 , from 298.15 to 318.15 K.

In the Eq. (4) Km is the equilibrium constant, mt is the total concentration in mol L−1 , and ıobsd, ımon, and ımic are the observed chemical shift, the monomeric chemical shift, and the micellar chemical shift, respectively. The slope of the straight line, which approaches zero in the X-axis, yields the Nagg (Fig. 5). The Nagg found for [BisOct(MIM)2 ]Br2 was 15 at all the temperatures evaluated. Zhao et al. [31] observed an Nagg of 25 when investigating analogous monocationic ([OctMIM]Br). Thus, changes in the Nagg for the dicationic IL [BisOct(MIM)2 ]Br2 , as a function of temperature, were not observed via 1 H NMR (Fig. 5). This means that the NMR technique is not sensitive to recognizing variations in the aggregate size as a function of temperature. This can be attributed to the very small variations in the chemical environment of the aggregate phase when the temperature is changed, which promote minor variations in the chemical shifts and, therefore, an unremarkable variation in Nagg as a function of temperature. 3.2. Effect of temperature on aggregate size DLS was used to investigate the size of aggregates as a function of temperature. The aggregate size of [BisOct(MIM)2 ]Br2 in water was examined at a concentration of 300 mM, between 298.15 and 318.15 K. One type of aggregate (Rh ) was found for all the temperatures evaluated (Fig. 6). The Rh values can be seen in Table 3. It was previously suggested that the degree of counter-ion binding (ˇ) to the micelle is dependent on the size of the aggregate. A larger

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G◦ agg data obtained through conductivity [15,16], which is shown in Eq. (8) [35]: G◦ ads = G◦ agg −

Fig. 7. Plot of surface tension versus ln C for [BisOct(MIM)2 ]Br2 in water, from 288.15 to 308.15 K.

aggregate has a greater predisposition for attracting counter-ions when compared to a smaller one. As can be seen in Table 1, the ˇ values of [BisOct(MIM)2 ]Br2 increase with the decrease in temperature, which indicates that larger aggregates are expected at lower temperatures. This assumption is confirmed by the DLS data. The Rh value decreases between 298.15 and 318.15 K, as can be seen in Fig. 6. A similar result was found by Ao et al. [26]. when the aggregate size of [bis-nHexDoDecIM]Br2 decreased upon increasing the temperature. 3.3. Dependence of the aggregate’s surface activity on temperature Surface tension measurements were performed to investigate the surface activity of the IL at different temperatures. Fig. 7 shows the plot of surface tension () versus the natural logarithm of the concentration (C) of [BisOct(MIM)2 ]Br2 , from 288.15 to 308.15 K. The surface tension decreases with the increase in IL concentration, which indicates that IL molecules are initially adsorbed at the air/water interface [32]. The formation of the aggregate can be detected when there is a change in the slope of  versus ln C. The CAC value was determined from this slope change [32]. Several parameters related to the surface properties of [BisOct(MIM)2 ]Br2 were obtained and they are listed in Table 4. The surface pressure at the CAC was defined by Eq. (5): CAC = ␥0 − ␥CAC

(5)

in which  0 is the surface tension of pure solvent, and  CAC is the measured surface tension at the CAC [33]. The maximum excess surface concentration ( max ) was estimated by applying the Gibbs adsorption equation to the surface tension data [32,34]. The prefactor “m” in the Gibbs equation, was assumed to be 2 for our gemini imidazolium IL with flexible methylene spacers — Eq. (6) [34]: max =

−1 d nRT d ln C

(6)

in which R is the gas constant (8.314 J mol−1 K−1 ), T is the absolute temperature, C is the IL concentration, and d/dln C is the slope in the surface tension isotherm. The minimum area, Amin (m2 ), occupied by a single surfactant molecule at the air–water interface was estimated using Eq. (7) [19]: Amin =

1 NA max

(7)

in which NA is the Avogadro constant and max in mol m−2 . The Gibbs free energy of adsorption (G◦ ads ) was calculated using

cac max

(8)

From the CAC values, it can be seen that an increase in temperature makes the aggregation process of [BisOct(MIM)2 ]Br2 in water more difficult. Furthermore, the surface tension at the CAC ( CAC ) also decreases—from 288.15 to 308.15 K, which indicates that the adsorption of the IL at the air/water interface improves with the increase in temperature. The minimum area occupied by a single IL molecule at the air/water interface, Amin , was estimated from the max value, and it is in agreement with methods previously described in the literature [33]. It can be seen in Table 4 that the higher temperature results in a greater Amin and a lower maximum excess surface concentration ( max ). According to Li. et al. [14] this may be due to the increased molecular motion at higher temperatures, which enables the adsorption of fewer molecules at the interface (lower max ), thus reducing the packing density. This tendency is in agreement with the surface pressure at the CAC (ПCAC ). This parameter measures the effectiveness of the surfactant to reduce the surface tension of the solvent. Thus, a smaller number of IL molecules are adsorbed at the interface (lower max ) and reduce the surface tension less effectively at higher temperatures, which is demonstrated by the temperature effect in this system. All the G◦ ads values found for [BisOct(MIM)2 ]Br2 were negative, which suggests that the IL adsorption at the air/water interface is spontaneous. Furthermore, G◦ ads was more negative than G◦ agg for all the temperatures, which indicates that the adsorption of the IL at the air/water interface is more favorable than the aggregation process. 3.4. Comparison of thermodynamic parameters for the aggregation of ILs Our research group has previously stated that [BisOct(MIM)2 ]Br2 exhibits higher CAC values than [Oct(MIM)]Br when surface tension and conductivity is taken into account. However, this time the CAC observed for [BisOct(MIM)2 ]Br2 through conductivity at 298.15 K (125 mM) was lower than the CAC described previously for the same IL (230 mM) [15]. The CAC detected for [BisOct(MIM)2 ]Br2 was comparable to the monocationic analogous IL [Oct(MIM)]Br (150 mM) [36]. The lower CAC is probably a consequence of Carpena’s method, which was used to determine the CAC. This fact also results in a greater difference between the CAC from conductivity and the surface tension of [BisOct(MIM)2 ]Br2 at 298.15 K. The CAC values for [BisOct(MIM)2 ]Br2 in [15] — which were determined via conductivity, surface tension, and fluorescence at 298.15 K — are notably different from the CAC values determined in this work via conductivity, surface tension, and NMR. These changes in the values can be related to the different techniques, methods and physical–chemical parameters monitored. The surface tension measurements described here were 20% greater than the previous measurements. This difference is acceptable for the surface tension measurement, because in the liquid–air interface, perturbation in this system caused by any residual traces can alter the CAC. The conductivity measurements shown here were about 46% lower than the previous measurements. In this case, the difference in CAC was expected because in this work Carpena’s method was used to find the CAC, while in the previous work [15] we used the conventional method. However, the value found in this work via conductivity (125 mM) is very close to the CAC value detected previously via fluorescence (135 mM) [15]. Carpena’s method is more suitable than the conventional method, because it does not depend on the judgment of the researcher and, consequently, it provides a

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Table 4 Surface properties of the IL [BisOct(MIM)2 ]Br2 in water. T (K)

CACa (mM)

 CAC b (mN m−1 )

max c (␮mol m−2 )

Amin d (Å2 )

CAC e (mN m−1 )

288.15 298.15 308.15

287 311 354

37.4 36.4 35.4

0.70 0.66 0.54

235.0 249.0 308.8

36.11 35.56 34.96

a b c d e f

G◦ ads f (kJ mol−1 ) −67.92 −69.94 −81.38

Critical aggregation concentration. Surface tension at CAC. Maximum excess surface concentration. Minimum area occupied by a single IL molecule at the air/water interface. Surface pressure at CAC. Gibbs free energy of adsorption.

Table 5 Thermodynamic aggregation parameters determined by the conductivity and surface properties of [Oct(MIM)]Br in water [36]. T (K)

ˇ

CAC (mN m−1 )

max (␮mol m−2 )

Amin (Å2 )

CAC (mN m−1 )

G◦ agg (kJ mol−1 )

H◦ agg (kJ mol−1 )

TS◦ agg (kJ mol−1 )

298.15 303.15 313.15

0.630 0.628 0.624

28.7 27.0 28.8

2.7 2.7 2.6

60 63 64

44.9 44.0 40.6

−24.8 −25.1 −25.8

−1.79 −1.84 −1.97

22.98 23.28 23.86

systematic application procedure. Lastly, higher CAC values found via NMR (468 mM) are expected, in accordance with the differences between values already presented in the literature [13]. Vaghela et al. [36] investigated the aggregation behavior of [Oct(MIM)]Br at 298.15, 303.15, and 313.15 K via surface tension and conductivity measurements (Table 5). The CAC found by conductivity was 150, 150.7 and 152.1, respectively thus, a comparison can be made, at different temperatures, between the aggregation behavior of this dicationic IL and its monocationic analog. In general, the same tendency of the CAC values to increase and the ˇ values to decrease when the temperature increases, was observed for dicationic and monocationic ILs. At 298.15 K, the ˇ value for dicationic IL (0.61) is lower than for the monocationic IL analog (0.63). This indicates that aggregates formed by dicationic ILs are less efficient in attracting counter-ions onto their surfaces. The active surface parameters, obtained by surface tension, demonstrate that the CAC and Amin increase, while ПCAC and max decrease when the temperature increases for monocationic (Table 5) and dicationic IL (Table 4). From the max values at 298.15 K, it is possible to observe that the dicationic IL has a greater tendency to adsorb at the water–air interface. The negative G◦ agg of dicationic (Tables 1 and 2) and monocationic (Table 5) IL indicates that the aggregation of these ILs is a spontaneous process. In general, dicationic IL has less negative G◦ agg values (Table 1). This suggests that the aggregation process of [BisOct(MIM)2 ]Br2 is less favorable than the monocationic analog, which agrees with the max data. The slightly negative enthalpy values for [Oct(MIM)]Br indicate that the aggregation of this IL is entropy-driven and that hydrophobic forces are predominant. In this work, it was shown that the aggregation process of [BisOct(MIM)2 ]Br2 in water was enthalpy-driven for all the temperatures evaluated.

4. Conclusion In this paper, results concerning the effect of temperature on the aggregation behavior of [BisOct(MIM)2 ]Br2 in water were investigated. The aggregation was an exergonic process (G◦ agg < 0), and it becomes less favorable with the increase in temperature. The aggregation of [BisOct(MIM)2 ]Br2 is enthalpy-driven (proven by H◦ agg ), which means that aggregation in dicationic IL is controlled more by the decrease in the electrostatic repulsion between the cationic heads than the increase in repulsion between alkyl chains (entropy-driven). The surface properties demonstrate looser packing aggregates and surface activity of IL at higher temperatures. The 1 H NMR provides the aggregation number and showed that aggre-

gates of dicationic IL in water are formed from a lower number of monomers in relation to monocationic ILs. A comparison between the thermodynamic properties of the aggregation of dicationic IL and its monocationic analogs reveals that the aggregation of monocationic IL is more favorable, while dicationic ILs have a tendency to adsorb at the water–air interface. Furthermore, it was observed that the aggregation of [Oct(MIM)]Br and [BisOct(MIM)2 ]Br2 is entropyand enthalpy-driven, respectively. These features are valuable for a better rationalization in relation to the temperature range for the stated IL applications. Acknowledgements The authors are grateful for the financial support from: the National Council for Scientific and Technological Development (CNPq) (Universal/Proc. 474895/2013-0; the Rio Grande do Sul Foundation for Research Support (FAPERGS) (Proc. 2262-2551/141 and 2290-2551/14-1); and the Coordination for Improvement of Higher Education Personnel (CAPES/PROEX). Fellowships from CNPq (M. A. P. M., C. R. B., and P. R. S. S.) and CAPES are also acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa.2016.01. 015. References [1] C.P. Frizzo, A.Z. Tier, C.R. Bender, I.M. Gindri, M.A. Villetti, N. Zanatta, et al., Structural and physical aspects of ionic liquid aggregates in solution, in: Scott Handy (Ed.), Ionic Liquids—Current State of the Art, 1st ed., InTech, Rijeka, 2015, pp. 1–40, http://dx.doi.org/10.5772/59287. [2] L. Douce, J.-M. Suisse, D. Guillon, A. Taubert, Imidazolium-based liquid crystals: a modular platform for versatile new materials with finely tuneable properties and behaviour, Liq. Cryst. 38 (2011) 1653–1661, http://dx.doi.org/ 10.1080/02678292.2011.610474. [3] I.M. Gindri, C.P. Frizzo, C.R. Bender, A.Z. Tier, M.A.P. Martins, M.A. Villetti, et al., Preparation of TiO2 nanoparticles coated with ionic liquids: a supramolecular approach, ACS Appl. Mater. Interfaces 6 (2014) 11536–11543, http://dx.doi.org/10.1021/am5022107. [4] B.M. Godajdar, B. Ansari, Preparation of novel magnetic dicationic ionic liquid polymeric phase transfer catalyst and their application in nucleophilic substitution reactions of benzyl halides in water, J. Mol. Liq. 202 (2015) 34–39, http://dx.doi.org/10.1016/j.molliq.2014.12.009. [5] A. Beiraghi, M. Shokri, S. Seidi, B.M. Godajdar, Magnetomotive room temperature dicationic ionic liquid: a new concept toward centrifuge-less dispersive liquid–liquid microextraction, J. Chromatogr. A 1376 (2015) 1–8, http://dx.doi.org/10.1016/j.chroma.2014.12.004.

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